Currently available telescopic cylinders typically include single and double acting types that are extendible upon the introduction of pressurized fluid. Fluid enters a port in an outer cylinder and continues through passages within multiple internal stages of the cylinder. These stages are comprised of coaxial piston/sleeve and plunger assemblies that move relative to the outer cylinder. Erratic fluid flow and trapped air within the cylinder provide obstacles to a desired smooth extension and retraction of the cylinder.
In certain prior art constructions pertaining to telescopic devices, such as U.S. Pat. No. 5,072,811 to Everhard; U.S. Pat. No. 5,322,004 to Sims, and DE Published Application. No. 2,004,117 to Nummi Oy, fluid travels between stages through radial apertures in the sleeve, or tube portions of the assemblies. One disadvantage of these constructions includes weakening the strength of the assemblies by having these apertures extending radially through the thin sleeves. The present invention places fluid apertures or passages in the pistons, which are of greater radial extent than the sleeves. This not only provides the present invention a structurally sounder assembly, but also allows for an orifice at an end of the aperture to be shaped or contoured, thus providing a more laminar flow.
Trapped air within the cylinder can cause sponginess, due to the compressability thereof, that is detrimental to the extension or retraction of the cylinder. In other prior art constructions, such as U.S. Pat. No. 732,142 to Tuggle et al., the cylinder bleeds trapped air out to the atmosphere through apertures in each stage. The present invention is designed without a bleeding orifice or valve due to the minimal amount of air trapped inside the cylinder. This bleederless feature is possible due to the low volume of fluid inside the cylinder and efficient sealing that prevents air from entering the cylinder. The volume of fluid within the cylinder has been reduced since the annular volume between the several stages is kept to a minimum. U.S. Pat. No. 2,692,584 to Armington et al. discloses a telescopic cylinder with a large annular volume between stages which has the potential of trapping more air in the system.
Prior art designs have improved laminar fluid flow in the cylinder by selectively removing material from the sealing rings or sliding bearings on the piston heads so that fluid can more readily flow from one end of the piston to the other. Parker Hannifin Corporation, the assignee of the present invention, uses bearings with circumferentially spaced longitudinal grooves so that the fluid pressure drop, from one side of the piston to the other is reduced. This type of bearing design is well known in the art.